Editor’s note: Today’s blog post come courtesy of the team at the Deimos Sky Survey (DeSS), who use a high-tech automated observatory located on top of Puerto de Niefla, in Valle de Alcudia and Sierra Madrona Natural Park, in central Spain, south of Madrid.
The location offers spectacularly clear skies and the observatory comprises three telescopes that are used for testing and development of software, systems and techniques for surveying asteroids and space debris, as well as actual detection and tracking of asteroids. The telescopes are operated remotely from a control room located at the Elecnor Deimos Castilla La Mancha facilities in Puertollano, about 35 km to the north. The Deimos Space Surveillance & Tracking team analysed the images, which were acquired in mid-January, when Tiangong-1 was at about 280 km altitude.
Noelia Sánchez Ortiz, Elecnor Deimos Director of Space Situational Awareness, wrote:
The images were obtained by our robotic telescopes commanded from the Deimos Sky Survey control centre, and were generated specifically by the ‘Antsy’ optical sensor, which is adapted for tracking objects in low-Earth orbits.
The imaging process is fully automated, both in the tasking of the sensor to point to the predicted position of Tiangong-1 and in the processing of the telescope images obtained. Due to the particular difficulties of observing an object at such low altitude, the imaging was monitored by an on-duty operator.
Images were acquired on 15 and 16 January 2018, during the approximately 1-minute satellite pass over our DSS location in south-central Spain. We used very short exposures, down to 10 miliseconds.
There’s a nice update today from Spacecraft Operations Engineer Armelle Hubault, working on the ExoMars TGO flight control team at ESOC.
TGO aerobraking visualisation to March 2018. Credit: ESA
This graphic (above) gives a very concise visualisation of the fantastic progress we’ve made with aerobraking to date.
It was coded by my ExoMars TGO colleague Johannes Bauer; the bold grey lines show successive reductions in the ExoMars TGO orbital period by 1 hour; the thin lines by 30 mins.
We started on the biggest orbit with an apocentre (the furthest distance from Mars during each orbit) of 33 200 km and an orbit of 24 hr in March 2017, but had to pause last summer due to Mars being in conjunction.
We recommenced aerobraking in August 2017, and are on track to finish up in the final science orbit in mid-March 2018. As of today, 30 Jan 2018, we have slowed ExoMars TGO by 781.5 m/s
For comparison, this speed is more than twice as fast as the speed of a typical long-haul jet aircraft.
On Tuesday this week at 15:35 CET, the spacecraft was where the red dot is, coming out of pericentre passage (passing through the point of closest approach over the surface – where Mars’ thin, uppermost atmosphere drags on the craft the most to give the braking effect).
The blue line is the current orbit, which takes only 2 hrs and 48 min and with the apocentre reduced to 2700 km; the red shows the final aerobraking orbit we expect to achieve later in March. Then, we will use the thrusters to manoeuvre the spacecraft into the green orbit (roughly 400 km circular) – the final science and operational data relay orbit.
The image is pretty much to scale.
We have to adjust our pericentre height regularly, because on the one hand, the martian atmosphere varies in density (so sometimes we brake more and sometimes we brake less) and on the other hand, martian gravity is not the same everywhere (so sometimes the planet pulls us down and sometimes we drift out a bit). We try to stay at about 110 km altitude for optimum braking effect.
To keep the spacecraft on track, we upload a new set of commands every day – so for us, for flight dynamics and for the ground station teams, it’s a very demanding time!
When TGO skims through the atmosphere, it has to adopt a specific orientation to optimise the braking effect and to make sure it stays stable and does not start to spin madly, which would not be optimal.
We are basically using the solar panels as ‘wings’ to slow us down and circularise the orbit.
Tracking aerobraking progress. Credit: ESA
The main challenge at the moment is that, since we never know in advance how much the spacecraft is going to be slowed during each pericentre passage, we also never know exactly when it is going to reestablish contact with our ground stations after pointing back to Earth.
We are working with a 20-min ‘window’ for acquisition of signal (AOS), when the ground station first catches TGO’s signal during any given station visibility, whereas normally for interplanetary missions we have a firm AOS time programmed in advance.
With the current orbital period now just below 3 hrs, we go through this little exercise 8 times per day!
FAQ prepared and updated by the Space Debris Office, ESA/ESOC, Darmstadt, Germany.
Tiangong-1 (天宫一号, Heavenly Palace 1) is China’s first space station and an experimental space laboratory. Its major goal was to test and master technologies related to orbital rendezvous and docking. It is identified by its UN COSPAR ID 2011-053A. It was launched on 30 September 2011 at 03:16:03.507 UTC by a Long March 2F/G rocket from the Jiuquan Satellite Launch Centre in the Gobi desert, Inner Mongolia, China. One uncrewed and two crewed missions, executed by the Shenzhou (神舟, Devine Craft) spacecraft, took place during its operational lifetime.
The Tiangong-1 space station will reenter Earth’s atmosphere and substantially burn up in the March–April 2018 timeframe.
As of mid-January 2018, the spacecraft was at about 280 km altitude in an orbit that will inevitably decay; it will mostly burn up due to the extreme heat generated by its high-speed passage through the atmosphere (some spacecraft, like Soyuz capsules, are designed to withstand reentry).
Following launch in 2011, the Tiangong-1 orbit began steadily decaying due to the faint, yet not-zero, atmospheric drag present even at 300 or 400 km altitude. This affects all satellites and spacecraft in low-Earth orbit, like the International Space Station (ISS), for example.
Tiangong-1 space station. Credit: CMSE/China Manned Space Engineering Office
As a result, such craft must conduct regular ‘reboost manoeuvres’ to maintain their orbit – typically, ground controllers command the craft’s engines or thrusters to fire for a certain amount of time, speeding it up so that it gains altitude.
During its operational life from launch through to December 2015, regular orbital maintenance manoeuvres were executed by Tiangong-1 in order to maintain an operational altitude of between 330 and 390 km above the Earth’s surface.
Q. What was the original disposal plan?
Initially, a ‘controlled reentry’ was planned for the spacecraft at the end of its life.
This means that ground controllers would have commanded the engines to fire, slowing the craft by a significant amount so that it would fall toward the surface. Firing the engines would have been done at a specific moment so that it would reenter the atmosphere and substantially burn up over a large, unpopulated region of the South Pacific ocean. Any surviving pieces would fall into the ocean, far from any populated areas. This is precisely what ESA did, for example, for the Agency’s series of five ATV cargo spacecraft between 2008 and 2015.
However, in March 2016 the Tiangong-1 space station ceased functioning but maintained its structural integrity. In so far as can be fully confirmed, ground teams lost control with the craft, and it can no longer be commanded to fire its engines. It is, therefore, expected to make an ‘uncontrolled reentry.’
Q: How big is Tiangong-1? What shape is it?
The spacecraft’s 10.4 m-long main body is made up of two cylinders of approximately equal length: a service module and an experiment module. The thinner service module provides power and orbit control capabilities for the station. It has two solar panels, each approximately 3 x 7 m in size. The thicker experimental module comprises an enclosed front conical section, which include a docking port, a cylindrical section, and a rear conical section. The experimental module is habitable.
This vivid image shows China’s space station Tiangong-1 – the name means ‘heavenly palace’ – and was captured by French astrophotographer Alain Figer on 27 November 2017. It was taken from a ski area in the Hautes-Alpes region of southeast France as the station passed overhead near dusk. The station is seen at lower right as a white streak, resulting from the exposure of several seconds, just above the summit of the snowy peak of Eyssina (2837 m altitude). Credit: A. Figer. Used by permission.
The overall mass of the spacecraft was reported to have been approximately 8.5 tonnes including fuel at launch. Given that the space station exceeded its originally planned operational lifetime of two years and continued operating successfully for two more years after that, a considerable amount of fuel must have been consumed to sustain the orbit and the habitable environment conditions inside.
This means that a significantly lower mass on reentry is likely, comparable to the mass of defunct satellites that make uncontrolled reentries typically a couple times per month.
Q. To date, who’s done or is doing what?
China notified the United Nations Office for Outer Space Affairs (UNOOSA) of the upcoming re-entry and committed to enhanced monitoring and forecasting of the orbital decay, including requesting an international joint monitoring and information dissemination campaign under the framework of the Inter-Agency Space Debris Coordination Committee (IADC).
IADC comprises space debris and other experts from 13 space agencies/organisations, including NASA, ESA, European national space agencies, JAXA, ISRO, KARI, Roscosmos and the China National Space Administration.
IADC members will use this event to conduct their annual reentry test campaign, during which participants will pool their predictions of the time window, as well as their respective tracking datasets obtained from radar and other sources. The aim is to cross-verify, cross-analyse and improve the prediction accuracy for all members.
ESA is acting as host and administrator for the campaign, as it has done for the twenty previous IADC test campaigns since 1998. A special case for ESA was the campaign in 2013 during the uncontrolled reentry of ESA’s own GOCE satellite.
As of January 2018, the mean altitude of the space station is 280 km. The further decay, and hence re-entry, is assumed to be uncontrolled in the sense of orbit maintenance. This has, however, not been unambiguously confirmed by the Chinese authorities. It has however been reported that the attitude, i.e. the orientation, of Tiangong-1 is stabilised.
Q. Over which parts of Earth will it burn up?
Due to the orbital inclination of the Tiangong-1, approximately 42.8 degrees, and the likely uncontrolled nature of the reentry, the final impact point can be anywhere on Earth between 42.8 degrees North and 42.8 degrees South in latitude.
Map showing the area between 42.8 degrees North and 42.8 degrees South latitude (in green), over which Tiangong-1 could reenter. Graph at left shows population density. Credit: ESA CC BY-SA IGO 3.0
Due to the geometry of the craft’s circular orbit, the probability of a reentry at the maximum 42.8 degrees N) and minimum (42.8 degrees S) latitude are higher than at the equator (roughly speaking).
Q. Will anyone know the precise location and time of reentry in advance?
Only from one day before the actual reentry will it become possible to roughly predict which ground tracks, and hence which regions on Earth, might witness the reentry.
But even then, an impact location prediction on the order kilometres is, for an uncontrolled reentry, beyond current technical capabilities due to complexities of modelling the atmosphere, the dynamics of the reentering object and limitations in observing the spacecraft.
In general, the uncertainty associated with an uncontrolled reentry prediction is on the order of 20% of the remaining orbital lifetime. Practically, this means that even 7 hours before the actual reentry, the uncertainty on the break-up location is a full orbital revolution – meaning plus or minus thousands of km!
The current reentry uncertainty window is shown below (the latest version will be posted in the home page of this blog).
Predicted time window for reentry. Horizontal axis shows the chart was generated. Vertical axis shows the range of dates during which reentry is most likely to occur. Credit: ESA CC BY-SA IGO 3.0
If the spacecraft does have a functioning attitude control system now, this could stop working under the higher dynamic pressure loads (due to falling lower into the atmosphere) closer to reentry and the uncertainty in the final reentry time window could rise (this was the case, for example, with ESA’s GOCE reentry).
Q. Once it reenters and breaks up, what is the risk that any pieces reach ground?
Tiangong-1 is a large spacecraft comparable in size and mass to other, frequently used space stations and cargo vessels such as ESA’s ATV, the Japanese HTV, Russian Progress and American Dragon or Cygnus.
From monitoring the controlled reentries of those types of spacecraft, it can be surmised that Tiangong-1 will break up during its atmospheric re-entry and that some parts will survive the process and reach the surface of Earth.
Video of ESA’s ATV 1 breaking up during its controlled reentry in September 2008
ATV-1 reentry - YouTube
Given the uncontrolled nature of this reentry event, the zone over which fragments might fall stretches over a curved ellipsoid that is thousands of kilometres in length and tens of kilometres wide. While a wide area could be affected, it is important to point out that a large part of the Earth is covered by water or is uninhabited.
Hence the personal probability of being hit by a piece of debris from the Tiangong-1 is actually 10 million times smaller than the yearly chance of being hit by lightning.
In the history of spaceflight, no casualties due to falling space debris have ever been confirmed.
Q. How does Tiangong-1 reentry compare to the reentries of similar-size craft in the past?
With its 8.5 metric tonnes of (initial) mass, Tiangong-1 is definitely not the largest uncontrolled reentry in spaceflight history. That would be Skylab with 74 metric tonnes.
Tiangong-1 falls within the category of modern space freighters (crewed and uncrewed) such as the already mentioned ATV (12 t), Japan’s HTV (10 t), Russia’s Progress (7 t) and Soyuz (7 t), the US Dragon (7 t) or Cygnus (5 t) and the Chinese Tianzhou (13 t). These masses are for the loaded craft; in the table below, they are shown at reentry.
Tiangong-1-class reentries Credit: ESA CC BY-SA 3.0 IGO Note: Shuttle Colombia (STS-107), with a mass of 82 t, unexpectedly broke up during a controlled reentry on 1 Feb 2003, leading to the loss of vehicle and crew.
Media and press seeking more information can contact the ESA Communication team as follows:
The current estimated window is ~17 March to ~21 April; this is highly variable.
Reentry will take place anywhere between 43ºN and 43ºS (e.g. Spain, France, Portugal, Greece, etc.). Areas outside of these latitudes can be excluded. At no time will a precise time/location prediction from ESA be possible.
Predicted time window for reentry. Horizontal axis shows the chart was generated. Vertical axis shows the range of dates during which reentry is most likely to occur. Credit: ESA CC BY-SA IGO 3.0
Current forecast altitude decay for Tiangong-1 Credit: ESA CC BY-SA 3.0 IGO
Editor’s note: This week’s blog update comes courtesy of TGO Spacecraft Operations Manager Peter Schmitz at ESA’s ESOC mission control centre in Darmstadt, Germany. The ExoMars Trace Gas Orbiter (TGO) has been conducting a complex and challenging aerobraking campaign since March 2017, using the faint drag of Mars’ upper atmosphere to slow it and lower it into its final science orbit, eliminating the need to have carried along hundreds of kilogrammes of fuel on its journey to the Red Planet. Aerobraking is expected to end around March 2018, after which TGO will perform some additional manoeuvres to achieve its final, near-circular, science orbit of about 400 km altitude.
Visualisation of the ExoMars Trace Gas Orbiter aerobraking at Mars. With aerobraking, the spacecraft’s solar arrays experience tiny amounts of drag owing to the wisps of martian atmosphere at very high altitudes, which slows the craft and lowers its orbit. Credit: ESA/ATG medialab
On Friday, 17 November, the flight controllers at ESOC began operations to bring the spacecraft into a new phase of the on-going aerobraking campaign, marking the start of ‘shorter’ orbits. ‘Short’ is considered, somewhat arbitrarily, as when the orbital period (i.e. time needed to complete one orbit) falls below 6 hrs.
Here’s a brief summary of progress to date.
TGO resumed its aerobraking campaign in August after a short break during summer due to conjunction with the Sun (that is, the Sun blocked the line-of-sight signal path between Earth and Mars), which makes for difficulties in communicating with the Red Planet.
Almost a month later, on 19 September, TGO’s operators faced, for the first time, a situation that violated the peak acceleration limits on the spacecraft, which then triggered an autonomous ‘flux reduction manoeuvre.’
During this operation, the propulsion system operated to raise the pericentre height (the point in the orbit where the spacecraft is closest to the planet) by 3 km, so that the next time the spacecraft passed through the atmosphere, the aerodynamic drag was reduced.
This event was quickly ‘recovered’ – which is engineer-speak meaning ‘everything got back to normal’ – so no delay in the overall aerobraking campaign was incurred.
“As of now, TGO aerobraking is on track with respect to our long-term predictions,” says Peter.
“On 8 November, our orbital period was seven hours and eight minutes, while now it is closer to six hours and twenty minutes.”
Another spacecraft anomaly occurred in the Data Handling System in October.
This time, the problem was caused by a corrupted ‘on-board control procedure’ (OBCP), which is a small program responsible for resetting the star tracker after any blinding condition (star trackers are cameras used to help determine the orientation of a spacecraft).
The operations team at ESOC noticed the problem when the autonomous execution of this procedure resulted in a checksum error – basically, an error indicating that a stored bit of data was not the value expected.
The situation was quickly recovered by re-uploading a fresh copy of the small OBCP programme.
However, mission controllers are, by nature, driven to fully understand the complex spacecraft for which they are responsible, and the event kicked off an intense investigation to determine why this anomaly occurred.
After a great deal of sleuthing work, the team found that this particular OBCP had been mistakenly overwritten by another command file, and that this problem could re-occur.
Although it was not a critical issue at the time it was discovered, the same malfunction could potentially overwrite more important command files or on-board control procedures that are required by the spacecraft’s computer for daily flight operations or contingency recovery situations.
“At first, we hypothesised that the failure could have been due to radiation effects on the spacecraft’s mass memory, or due to an on-board feature that corrects memory ‘bitflips’ automatically,” says Johannes Bauer, TGO’s data handling engineer.
A ‘bitflip’ occurs when a single stored data bit – a 1 or a 0 – randomly flips to the opposite value due to the passage of solar or cosmic radiation through the solid-state memory.
An investigation was launched and Johannes and the TGO team at ESOC worked together with the spacecraft manufacturer, Thales Alenia Space, to define and implement a fix.
“Initially, the problem was mitigated by uploading files in a certain order,” said Spacecraft Operations Engineer Chris White.
However, TAS rapidly coded a new software patch that was successfully tested and validated on the TGO simulator at ESOC and on the avionics test bench – basically, an engineering copy of the spacecraft’s flight control systems and computer – located at a TAS factory in Italy.
“The team is planning to upload the central software RAM patch to the spacecraft in the coming days, which should solve this problem,” says Chris.
Final phase of aerobraking
Despite these and a number of other smaller issues, TGO’s final aerobraking operations have already started and this involves both the space segment (i.e. the spacecraft) and the ground segment (i.e. the systems used on Earth to fly TGO).
This month, the flight control team will set the mission control system into a ‘hot redundant’ configuration – with two identical ground control systems working at the same time providing immediate back-up in case one control system becomes unavailable.
The team will also use an automated system configuration to open and close telemetry and telecommand links1, saving time during ‘live’ operations (when the team are in contact with the spacecraft via a ground station like ESA’s New Norcia station in Australia or Malargüe station in Argentina, or via a NASA deep-space network station) and reducing the chances of human error during delicate manoeuvres.
Time gets tight
From now on, the aerobraking campaign will gradually evolve.
TGO Aerobreaking schedule. Credit: ESA
The spacecraft will slow, increasing the height of its pericentre and, consequently, reducing the effect of the atmosphere’s drag on TGO. During this phase, it will orbit around Mars multiple times a day and operations will intensify.
This means that the flight control team and the flight dynamics specialists at ESOC will have to estimate the spacecraft’s orbits and upload new commands daily, instead of every two days as is the case now.
Until the end of aerobraking in March 2018, the daily commanding volume will steadily increase because more and more orbits will be flown per day while the available time to compose and transmit commands to the spacecraft will become tighter – primarily because the ground station contacts, or passes, will be increasingly interrupted whenever the spacecraft passes through the Red Planet’s atmosphere.
Aerobraking on track
As of now, the flight control team expect that the aerobraking campaign will conclude in March 2018, as planned.
However, there are still a few months to go, and unforeseen issues – or another flux reduction manoeuvre – could yet delay aerobraking progress.
“If aerobraking were to be delayed by a week, for example, that would surely affect our routine operations planning, but not so much the overall mission timeline,” says Peter.
The situation, would be different, however if any delay were to last longer.
“The implications are more severe, for example, if we had a delay of a month,” says Peter.
“Then, there would be knock-on effects in ground station scheduling with regard to other missions and flight control team engineer scheduling and assignments, and the start of the routine science and data-relay mission could be noticeably delayed.”
Aerobraking operations require 24-hour/day, 7 days/week ground station coverage, and at the moment TGO are using the two ESA stations mentioned earlier as well as NASA Deep Space Network stations.
“The ground-station booking schedule is agreed with other ESA and NASA missions. If TGO needed to extend its usage of ground stations for the final aerobraking phase, it would have effects on other missions, too, as their science return would be affected and operations need to be re-planned.”
“For now though, we are in good shape and everything is on track, and we’re looking forward to achieving our final science orbit and the start of data gathering and relay at Mars.”
Note: (1) ‘Telemetry’ is the on-board status information that the spacecraft transmits to ground, informing engineers of its current status and conditions, while telecommands are the instructions prepared on ground that are sent up by the flight controllers to tell the spacecraft what it should do.
We received an update from Robert Guilanya, flight dynamics lead for ExoMars/TGO, earlier today. He provided a short explanation of the Phobos orbit crossing that happened, for the first times, at 14:30 UTC and 20:00 UTC.
The crossing of the TGO orbit by the other Mars satellites is a normal situation that we monitor. During the last two weeks, we have been controlling the progress of the aerobraking campaign such that at the time Phobos crosses the TGO orbit, TGO is as far away as possible.
For today’s orbit, see below a series of plot that shows the Phobos orbit (blue line) and the TGO orbit (black line). The red dot shows Phobos’ position, and the black dot, TGO’s position.
To give you some numbers with the orbit of today:
Today at 06:44 UTC Phobos crossed the TGO orbit
Phobos orbit & TGO trajectory 06:44 UTC 16 Nov 2017 Credit: ESA/R. Guilanya
259 min later, TGO crossed Phobos orbit, at 11:03Z
Phobos orbit & TGO trajectory 11:03 UTC 16 Nov 2017 Credit: ESA/R. Guilanya
200 min later, Phobos crossed again the TGO orbit, at 14:23Z
Phobos orbit & TGO trajectory 14:23 UTC 16 Nov 2017 Credit: ESA/R. Guilanya
As you can see, both satellites (Phobos, too, is a ‘satellite’) had a large phase difference at the time they were crossing the orbits.
By the end of 2017, ESA will have transferred three stations to national organisations in Spain and Portugal, who will take over the provision of satellite tracking services to a wide variety of commercial customers.
The three stations involved in the transfer are all equipped with 15 m-diameter dish antennas, suitable for supporting near-Earth missions, and are located in Spain, at Maspalomas and at ESA’s space astronomy centre near Madrid, and in Perth, Western Australia.
The new operators will be able to use the stations to offer tracking services on a commercial basis to customers worldwide, which also includes ESA, leaving the Agency free to focus on meeting the demanding technical requirements of its deep-space stations, in Spain, Argentina and Australia, and on operation of a select group of four other stations.
ESA’s Lionel Hernandez, current Cebreros station manager and former manager for the ESAC antenna, provided some background on the station’s history.
On 1 September 2017, ESA’s VIL-2 antenna and its supporting facilities was formally retired after 36 years’ service supporting some of Europe’s most ambitious and successful missions.
Villafranca tracking station 1977 Credit: ESA
ESAC was inaugurated in May 1975 as the Vilspa station and has been responsible for providing telemetry, tracking and command support to not only ESA satellites but also to several agencies like NOAA, NASA, DLR (The German Aerospace Centre), the Chinese National Space Administration (CNSA) and to intergovernmental organisations such as Eumetsat.
It also supported missions flown by entities that later became private companies including Eutelsat, a French-based satellite communications provider and the UK’s Inmarsat plc (formerly INMARSAT), both leaders in global mobile satellite communications.
The station, now an ESA Establishment, continues to host the Science Operation Centres of almost all of ESA’s scientific missions.
From the early days of the International Ultraviolet Explorer (IUE) mission, the work now performed at ESAC continues to reach into the depths of space and across the electromagnetic spectrum; providing scientific data to the world.
What began as one of Europe’s first links to the stars has become the heart of European space science.
Satellites/missions supported by the Vilspa ground station
IUE, OTS-2, GOES-1, Marecs-A, Exosat, ECS-1, Marecs-B2, ECS-2, ECS-4,ECS-5, Olympus, Hipparcos, Giotto, Italsat-F1, ERS-1, Meteosat-4 (MOP-1), Meteosat-5 (MOP-2), Meteosat-6 (MOP-3), Meteosat-7(MOP-4), ISO, ERS-2, Italsat-F2, SOHO, XMM-Newton, Cluster ESA’s four-satellites flotilla, Envisat, Eutelsat-W3, MSG-1, Integral, SMART-1, Double Star Programme (TC-1 and TC-2 satellites) belonging to China National Space Administration, Bird, Meteosat-9 (MSG-2), ASTRA´s four Atlantic satellites fleet, MetOp-A and MetOp-B.
It has also supported Jules Verne (ATV-1), Johannes Kepler (ATV-2), Edoardo Amaldi (ATV-3), Albert Einstein (ATV-4), Georges Lemaitre (ATV-5) and the International Space Station (ISS).
Editor’s note: Today’s update comes from ESA’s Armelle Hubault, a spacecraft operations engineer working on the ExoMars/TGO team at ESOC. The news? ESA’s ExoMars/TGO orbiter – now conducting a year-long aerobraking campaign at Mars – crossed the orbit of Phobos today (spoiler alert: we avoided a collision!), marking a notable milestone in progress toward attaining its final, ca. 400-km altitude circular science orbit.
Phobos seen by Hubble: While photographing Mars, The ESA/NASA Hubble Space Telescope captured a cameo appearance of the tiny moon Phobos on its trek around the Red Planet (click on image for full details). Credit: NASA/ESA/Z. Levay (STScI) – Acknowledgment: J. Bell (ASU) & M. Wolff (Space Science Institute)
Here are the facts about the Phobos orbit crossing today.
The orbit crossing is not a Phobos flyby. In fact, we did our best to ensure that Phobos would be at the farthest possible point away from TGO when we cross the moon’s orbit. The moon will basically be on the other side of Mars when our spacecraft crosses its orbit [Editor: Phobos will be 9320 km from the centre of Mars for the first crossing].
This results in two crossings today: one around 14:30 UT and a second at 20:00 UT (15:30 and 21:00 CET, respectively). On each crossing of Phobos’ orbit, TGO will ‘miss’ the Phobos orbit by 23 km (and 120 minutes) and 10 km (and 200 minutes), respectively.
Visualisation of the ExoMars Trace Gas Orbiter aerobraking at Mars. With aerobraking, the spacecraft’s solar array experiences tiny amounts of drag owing to the wisps of martian atmosphere at very high altitudes, which slows the craft and lowers its orbit. Credit: ESA/ATG medialab
Note that the diameter of Phobos is about 20 km, so these passes by the orbit are very, very close!
Over the last few days, we adapted the phase of our orbit to ensure maximum ‘outphasing’ of Phobos and TGO, so today there is actually nothing for the flight control team to do but watch and monitor.
The crossing is taking place around apocentre (point of farthest approach to Mars); remember that our pericentre (point of closest approach) remains on the order of 100 km from the martian surface, actually in the atmosphere, which is how we are obtaining the aerobraking effect.
ESA’s Gaia mission, in orbit since December 2013, is surveying more than a thousand million stars in our Galaxy, monitoring each target star about 70 times over a five-year period and precisely charting their positions, distances, movements and brightness.
Although Gaia is not equipped with a dedicated radiation monitor, it can provide information about space weather – and the solar particles and radiation – that it encounters at its unique orbital position, L2, 1.5 million km from Earth in the direction away from the Sun.
This information is useful for studies of the Sun and the interplanetary radiation environment that will be encountered by future missions beyond Earth’s protective magnetosphere.
Solar cycle 24 and recent activity
Solar cycle 24 is the current 11-year period of varying activity which peaked in 2014; it has been notable because the Sun has been much quieter in this cycle than in the preceding ones. In September 2017, however, the Sun burst into life, erupting with the largest flare of the current cycle followed a few days later by another large flare. These flares were associated with a sunspot region with a complex magnetic structure (see image below) and were combined with a release of high-energy particles and coronal mass ejections.
Sunspots imaged in H-alpha light prior to flare activity in September 2017 with a blue dot showing the size of Earth for comparison. Region 2673 is visible at top right. This region can be seen to be magnetically complex (twisty) and dense/energetic. Credit: ESA/E.Serpell
According to spaceweatherlive.com these flares, which are observed and characterised in optical and x-ray wavelengths, had peak magnitudes1 of X9.3 and X8.2 and were the 8th and 11th largest observed since June 1996 respectively (see also @esaspaceweather here and here). It is interesting to note that with two entries in the top 11, the responsible region was perhaps the second most active of the last 21 years. This is rather surprising considering the previously observed nature of cycle 24 and the timing within this cycle.
Gaia as a particle detector
Gaia is continuously exposed to a stream of charged particles mostly emitted by the Sun, with the addition of some cosmic rays from much further away.
When charged particles pass through the CCD pixels of the spacecraft’s camera, they leave trails of charge that are read out from the detector. Depending on the angle between the direction of flight of the particle and the focal plane, there can be particle trails created with lengths from single pixels up to several hundred pixels.
The onboard computers of the camera are programmed with an algorithm that identifies these signals as prompt particle events (PPE), so called because the particles are high energy and therefore fast (prompt). The computers accumulate counters of these events, which are regularly sent to the ground allowing a measurement of the PPE rate to be calculated.
A WFS snapshot captured during normal space weather conditions showing multiple images of a bright star in a grid pattern and a small number of particle tracks (light dots). Credit: ESA
To assess the alignment of the on-board telescopes, Gaia is equipped with a pair of wavefront sensors (WFS; essentially, small cameras) that take snapshot pictures of specially imaged bright stars. Sometimes the WFS can be tricked by the bright signal due to a prompt particle track and a snapshot of this track is captured and transmitted to the ground (see figure above). These WFS snapshots contain a wealth of data about the particles that left the tracks, including incident direction and kinetic energy.
Gaia observations of a large flare in September 2017
The largest particle flux measured by Gaia since launch was detected after the magnitude X8.2 flare that peaked in x-rays at 16:06 (all times UTC), 10 September 2017. The first particles are apparent in the Gaia PPE counters at about 16:20 and the first WFS images start to appear from about 16:40. The signal peak occurred in both Gaia data sets at about 00:00 on 11 September and continued until at least 12:00 on 12 September (see charts below).
PPE counts (top) and WFS energy measurements (bottom) showing the high-energy particle environment around Gaia due to a solar flare in September 2017. Credit: ESA
Particle energies are measured in units of electron-volts, eV, and high-energy protons have a velocities that are significant fractions of the speed of light, c, as listed below (with the time of travel from the Sun to Gaia);
1MeV : 0.05c (180 minutes)
10MeV : 0.14c (57 minutes)
100MeV : 0.43c (19 minutes)
1000MeV : 0.87c (9 minutes)
Although it is not known at what point during the flare the first particles were released toward Gaia, the time of arrival after the peak (< 1 hour) is consistent with the energies (>10MeV) reported by the US GOES spacecraft, which is equipped with specific particle instrumentation that monitored this event from Earth orbit.
The WFS algorithm can be tricked to take an image of high energy particles instead of bright stars. Multiple particle tracks of varying lengths were captured in this example WFS snapshot near the peak of measured activity following a solar flare in September 2017. Credit: ESA/E. Serpell
The intensity of the flare can be appreciated by inspecting WFS snapshots taken near to the observed peak (see figure above) in comparison to images from quieter periods.
There is a relationship between the energy loss as a particle passes through the detector and the kinetic energy of the particle. From sufficiently long tracks it is possible to measure the energy of individual particles and it is hoped that in future an energy spectral analysis of the event will be computed.
The spectacular track in the example WFS snapshot above, if it was a proton, would have had a kinetic energy of 375MeV.
(1) Solar flares are classed according to the energy they release at X-ray wavelengths. There are five major categories: A, B, C, M and X, further divided into 10 subclasses. M1 flares are 10 times more powerful than C1, and X1 flares are 10 times more powerful than M1 flares, or 100 times more powerful than C1.
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